Before it was built in the 19th century, there was no simple way to get to the depths of Siberia. Today, it still acts as the main form of transportation between many Russian towns, including the rural Vladivostok and Irkutsk.

"There aren’t many flights and they’re very expensive for ordinary Russians," says Wolmar. "And the roads are just too long."

Wolmar tells us more of the story behind the 5,700 mile long railway in the audio player above.